1. Field of the Invention
The present invention relates to methods for continuously manufacturing low-oxygen copper, containing a suppressed level of oxygen content, by continuously casting molten copper produced in a melting furnace.
2. Description of the Related Art
Low-oxygen copper (called “oxygen-free copper” in some cases) in which the content of oxygen is controlled to 20 ppm or less, and more preferably, to 1 to 10 ppm, is widely used for producing various shapes, e.g., ingot forms such as billets and cakes, rolled sheets, wires and cut forms. As a method for manufacturing low-oxygen copper, a method is typically used in which molten copper is produced in a high-frequency furnace such as a channel furnace or a coreless furnace, the molten copper is transferred to a continuous casting machine while held in an airtight atmosphere and the casting is then performed.
When low-oxygen copper is produced by using a high-frequency furnace as described above, there are advantages in that a higher temperature can be easily obtained by a simple operation, and the qualities of the products are very uniform since no chemical reaction occurs in production of the molten copper. However, there are disadvantages in that the construction cost and the operating cost are high, and productivity is low.
In order to perform mass production of low-oxygen copper at lower cost, a method using a gas furnace such as a shaft kiln is preferably employed. However, when such a gas furnace is used, since combustion is performed in the furnace, i.e., oxidation occurs, the oxidized molten copper must be processed by a reducing treatment. This is the disadvantage of the gas furnace, which is not observed when a high-frequency furnace is used. As a result, low-oxygen copper cannot be produced unless oxygen contained in the molten copper is decreased by using a reducing gas and/or an inert gas in a step of transferring the molten copper before the molten copper is fed to a continuous casting machine.
In addition, even when the deoxidizing step described above is performed, holes will be formed in the low-oxygen copper and may result in generating defects such as blisters in some cases. As a result, the quality of the low-oxygen copper is degraded. In particular, when copper wire is manufactured, the holes described above will cause defects in a rolling step, and hence the copper wire has poor surface qualities. Accordingly, in general, it is believed that production of high quality low-oxide copper is difficult to perform using a gas furnace, and hence most of low-oxide copper is produced using a high-frequency furnace.
The holes described above are formed by bubbles of steam (H2O) produced by combination of hydrogen and oxygen due to the decease in solubility of the gases in the molten copper when it is solidified. The bubbles are trapped in the molten copper during cooling and solidification and remain in the low-oxide copper, and hence holes are generated. From a thermodynamic point of view, the concentrations of hydrogen and oxygen in molten copper can be represented by the equation shown below.[H]2[O]=pH2O·K  Equation (A)
In the equation (A), [H] represents the concentration of hydrogen in molten copper, [O] represents the concentration of oxygen in molten copper, pH2O represents a partial pressure of steam in the ambience, and K represents an equilibrium constant.
Since the equilibrium constant K is a function of temperature and is constant at a constant temperature, the concentration of oxygen in molten copper is inversely proportional to the concentration of hydrogen. Accordingly, in accordance with the equation (A), the concentration of hydrogen is increased by performing a deoxidizing treatment by reduction, and as a result, holes are easily generated during solidification, whereby only an ingot of low-oxygen copper having poor quality can be manufactured.
On the other hand, molten copper containing hydrogen at a low concentration can be obtained by melting copper in a state near complete combustion using an oxidation-reduction method, which is a general degassing method. However, in a subsequent deoxidizing step, a long moving distance of the molten copper is required, and hence the method described above cannot be practically used.